Physics · Glossary

What is Gravitational potential energy?

Also known as: potential energy gravitational · reference level

Definition 9.4 High School Physics · Chapter 9 — Energy: Forms and Conservation

A body of mass $m$ at height $h$ (in $\mathrm{m}$ ) above a chosen reference level stores the gravitational potential energy $E_p = m g h$ , with $g = 9.81\,\mathrm{N}/\mathrm{kg}$ — valid near the ground, where $g$ is effectively constant. Only differences of height matter: the level where $E_p = 0$ is chosen freely.

Examples

Example 9.5 (Water tower)

Each cubic metre ( $1000\,\mathrm{kg}$ ) of water stored $30\,\mathrm{m}$ up a water tower holds $1000 \times 9.81 \times 30 \approx 2.9 \times 10^{5}\,\mathrm{J}$ — about six tenths of the car’s kinetic energy above. Cities stockpile energy overhead.

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Definition 18.6 High School Physics · Chapter 18 — Mechanical Energy and Its Conservation

A body of mass $m$ at altitude $z$ , measured upward from a chosen reference level where $z = 0$ , stores the gravitational potential energy

E_p = m g z, \qquad g = 9.81\,\mathrm{N}/\mathrm{kg}.

Examples

Example 18.8 (A book on a shelf)

A $1.2\,\mathrm{kg}$ book sits $1.8\,\mathrm{m}$ above the floor, itself $9.0\,\mathrm{m}$ above the street. Reference at the floor: $E_p = 1.2 \times 9.81 \times 1.8 \approx 21\,\mathrm{J}$ ; in the street: $E_p \approx 127\,\mathrm{J}$ . Falling to the floor releases $21\,\mathrm{J}$ in both accounts.

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